A monopulse antenna system is commonly used to implement radar tracking or to track intentional radiators. As its name implies, a monopulse antenna system employs a single pulse to identify the presence of an object in the field of view. This is possible due to the use of multiple antennas which can detect angle information from the arriving signal.
FIG. 1 illustrates an example block diagram of a typical dual-axis monopulse antenna system. As shown, a dual-axis monopulse antenna system may include four individual antennas (A, B, C, and D) each of which are driven by the same arriving signal. If an object is in the field of view of the monopulse antenna system, each individual antenna will receive the reflected (or transmitted) signal. These received signals (which are referred to as A, B, C, and D respectively) are then fed to a comparator network. Although not shown, some intermediate processing may be performed on the received signals prior to inputting them to the comparator network.
Due primarily to the slight differences in the positions/orientations of the individual antennas, the characteristics of the received signals will vary. The comparator network can detect these variations to allow the relative location of the object with respect to the boresight axis to be determined. In particular, the comparator network can generate three tracking channels: (1) a sum (Σ) of the four received signals; (2) an azimuth difference (Δaz); and (3) an elevation difference (Δel). As one of skill in the art would understand how these tracking channels can be employed to identify and track the position of an object, no further description will be provided.
As with most antennas, monopulse antennas produce a mainlobe (or main beam) and various sidelobes. In many situations, it will be possible to detect the presence of an object (or intentional radiator) whenever the object is positioned within the mainlobe or within one of the sidelobes due to the relatively high gain of some sidelobes. Therefore, even if the monopulse antenna is not pointed directly at the object, it may still receive a strong enough signal to detect the object's presence. However, if the object is within a sidelobe, and if the comparator network detects a sum channel peak or a difference channel null, the antenna will incorrectly assume that it is pointing directly at the object.
In typical monopulse antenna systems, an open-loop GPS and navigation data backbone is employed to perform coarse tracking. In other words, GPS data of the object to be tracked is supplied to the monopulse antenna system to allow the monopulse antenna system to initially point the antenna in the general direction of the object. Using GPS data in this way also requires that the antenna be physically oriented with respect to true north which can be a tedious process.
Additionally, as part of this tracking system, a modem lock signal will typically be employed as an indicator to the system that tracking is occurring. At sufficiently large target ranges, because the mainlobe gain is larger than the sidelobe gains, a SNR sufficient to establish the modem lock should only exist when the object is within the antenna's mainlobe. However, the fact that a modem lock can be established does not necessarily imply that the object is within the antenna's mainlobe. In many situations, an adequate SNR for establishing a modem lock may exist even though the object is positioned within the antenna's sidelobe. In such situations, the monopulse antenna system will track the object using a sidelobe when the desired outcome is to track within the mainlobe. This increases the risk of dropping the link due to marginal signal-to-noise ratio performance as the target moves away. Accordingly, a modem lock is a poor indicator of mainlobe tracking.